AST 455 Astronomical Interferometry

Credit Hours: 4
Prerequisite: AST 403 and PHY 415

This course is an introduction to the principal technique of modern radio astronomy, and an increasingly important tool for infrared and visible wavelengths: spatial interferometry. We discuss the elements of physical optics, coherence theory, and the physics of detectors and receivers that bear on astronomical interferometry. We follow this formal development with a detailed account of the practice of interferometry, calibration, and data reduction. The intention is to provide to students all they need to know to understand, plan, propose, and analyze observations with such instruments as the Very Large Array (VLA), the Very Long Baseline Array (VLBA), the Owens Valley Radio Observatory's (OVRO) Millimeter Array, and the Berkeley-Illinois-Maryland Array (BIMA) at Hat Creek Radio Observatory.

syllabus

1. Introduction and history. The need for high angular (spatial) resolution in astronomy; the history of the experimental approaches applied.
2. Simple theory of interferometry. Michelson's stellar interferometer: its operation, and the Fourier transform, relationship between fringe visibility measurements and the source, brightness distribution; modern implementations.
3. Formal theory of interferometry. Underlying assumptions. Spatial coherence: relationship between autocorrelation and power. Spectrum, and between complex degree of coherence and source brightness distribution: the Wiener-Khinchin relation and the Van Cittert-Zernike theorem. Temporal coherence: correlations between fluctuations in thermal emission and the basis for intensity interferometry; coherence length, coherence time. Two-telescope interferometers with correlators. Effects of bandwidth (space-frequency equivalence), primary beam shape, and geometry.
4. Interferometers and arrays. Electric field measurements with heterodyne receivers: modern heterodyne receivers; the mixing theorem; quantum noise and the sensitivity of receivers. Geometrical considerations in interferometers and telescope arrays: Coordinate systems; u-v plane sampling and the sampling (Nyquist) theorem; design of arrays. Response of the receiving system: frequency conversion and sideband separation; delay tracking; correlators, receiver noise and noise in a synthesized image. VLBI: special considerations; geodesy.
5. Calibration and transformation of interferometric data: "synthesis imaging". Calibration: complex gains; closure relations. Fourier inversion of the data: model fitting and direct inversion, and their undesirability; gridding; the Danielson-Lanczos lemma and the Fast Fourier Transform (FFT). Image processing: the CLEAN algorithm; maximum entropy and maximum likelihood deconvolution; self-calibration.
6. Interferometry at infrared and visible wavelengths. Intensity interferometers.
7. Infrared heterodyne interferometers. Speckle interferometry: origins; the Knox-Thompson algorithm; closure phases and the "bispectrum" at visible wavelengths. The wave of the future: modern Michelson-type interferometers.

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